1. Field of the Invention
The present invention relates to apparatus for extruding blown plastic film. More particularly, it relates to means for collapsing a tubular bubble of blown plastic film.
2. Prior Art
All blown plastic film is extruded either vertically, up or down, or horizontally, from an annular die. Once the plastic or polymer leaves the die, it forms a tubular xe2x80x9cbubblexe2x80x9d and is drawn from the die by means of two rollers (usually known as xe2x80x9cnip rollersxe2x80x9d) which contact a collapsed outer end of the bubble. As it leaves the die, the bubble is inflated with air by blowing means which supply air through the die, and once the requisite diameter has been reached inflation ceases and the air is trapped between the face of the die and the nip rollers.
When extruding blown film, the material in the form of a bubble is drawn from the die through the set of nip rollers, which are operated at variable speeds. This speed sets the final thickness of the film, which is dependent on how quickly the material may be drawn from the die. An apparatus commonly known as an air ring is mounted on top of the die. The air ring is designed to deliver a constant and precise flow of air from a pressure blower. This airflow reduces the temperature of the polymer sufficient to crystallise the molecules, which will set the final film diameter known as the freeze line or frost line.
Once the conditions are set, the bubble diameter is constantly being guided vertically through the bubble glides, and other guiding apparatus to the top nip. Immediately prior to the top nip, a set of collapsing frames mounted at an adjustable angle from the bubble diameter, at its entrance, to close to zero at the top, creates the transition of the bubble from cylindrical, to a tubular layflat sheet. This tubular layflat sheet is conveyed to the winding apparatus where the film can be rolled for further processing such as printing and bag making.
Current advances in technology including polymer chemistry, innovative cooling devices, and creative guiding devices, have resulted in increased extrusion or production speeds. With this combination of softer polymers and high extrusion rates, the collapsing frame technology, prior to the top nip has been a limiting factor, unable to handle the increased flow rates, resulting in deformed layflat sheet, creating unacceptable quality and reduced production rates of the finished product. Numerous methods have been tried to minimize this problem. Examples include Teflon roller collapsing frames, air collapsing frames, air tables, and tendency roller collapsing frames, all of which have had limitations, in either lack of quality or reduction in production capabilities.
Various so-called collapsing frames have been used in the prior art for guiding and collapsing the tubular bubble before it passes into the nip between the pinch rollers. Such frames originally consisted of wooden slats, rollers either driven or idle and driven by the film, xe2x80x9cTeflonxe2x80x9d (Trademark) rollers or beads, or bars or strips having air outlets to minimize contact with the soft plastic film. One problem with the usual types of collapsing frame is the difficulty of providing a smooth transition across the gap which exists between the outer end of the frame, where the transverse dimension of the tube is smallest, and the roller nip. This gap allows the film to expand slightly, under the pressure of air, and this in turn tends to cause wrinkles as the film is drawn into the nip.
Proposals have been made for using converging belts to provide, in effect, a part of the collapsing frame. So far as Applicant is aware, such proposals have not been adopted in practice. The problem with using converging belts is that any slight discrepancy between the surface speed of the belts will cause wrinkles as the blown tube enters the nip. The present invention overcomes this problem.
The present invention endeavours to provide an improved film-collapsing device. The device combines a collapsing frame and top nip assembly in order to overcome the quality and yield problems, blow film processors currently encounter, associated with the processing of newer softer polymers and the requirement for higher extrusion rates. This provides a means to control and optimize the collapsing frame""s operating parameters.
The present invention, utilizes two conveyors which converge and encompass the top nips. Rather than as conventionally occurring in two stages, the collapsing process occurs in one continuous flow. Collapsing from round to flat, the centre of the layflat tube moves with the conveyor while the outside of the tubular film will not drag, but will systematically be laid down on the surface of the conveyors. The upper drive rollers of the conveyors form the top nip.
The present invention endeavours to mitigate the following problems:
wrinkling during collapse;
use of expensive additives;
friction between top nip and collapsing frame;
film stretch between top nip and collapsing frame;
small collapsing angles (less than 30xc2x0); and
low production speeds.
In accordance with this invention, apparatus for producing plastic film of the type described, and having means for collapsing the bubble into a flat tube prior to passing the tube through a nip formed between rollers, is characterized in that the collapsing means includes opposed reaches of endless belts having their upstream ends held by a pair of first rollers spaced apart on opposite sides of the bubble, and having their downstream ends held by a pair of nip rollers which hold the belts together to form a nip for the collapsed tube; and in that drive means are provided which rotatably interconnect the pinch rollers and ensure that the belts entering said nip travel at the same surface speed.
The drive means may include a pulley mounted to rotate with each of the pinch rollers, and a single drive belt having an inside surface frictionally driving a first of the pulleys and having an outside surface frictionally driving the second of the pulleys. Alternatively the drive means may include a gear wheel mounted to rotate with each of the pinch rollers, the gear wheels being of the same size and meshing with each other to ensure that the pinch rollers rotate at the same speed.
The terms xe2x80x9cnip rollerxe2x80x9d and xe2x80x9cpinch rollerxe2x80x9d are used interchangeably in the description that follows.